1. Field of the Invention
This invention relates to a solid solution of R.sub.y Zr.sub.4 Si.sub.x P.sub.6-x O.sub.24 system (wherein R is at least one bivalent or trivalent element, x is a numerical value of 0 to 2, and y is a numerical value of 2/3 to 2 satisfying an electrically neutral condition of the chemical formula), a heat-resistant sintered body and a heat-resistant phosphate based compound sintered body each comprising the above solid solution, and a method of producing the same. More particularly, the present invention relates to a solid solution of R.sub.y Zr.sub.4 Si.sub.x P.sub.6-x O.sub.24 system having excellent heat resistance and high temperature stability, a heat-resistant high strength sintered body comprising the above solid solution and having low expansion and improved high temperature stability, a phosphate based compound sintered body comprising the solid solution and having excellent heat resistance and high temperature stability and low expansion, and a method of producing the same.